In recent years, a technique for forming a thin film transistor (TFT) by using a thin semiconductor film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been applied to a wide range of electronic devices like integrated circuits (ICs) and electro-optical devices, and development of thin film transistors as switching elements in image display devices, in particular, has been hastened.
Metal oxides have been known as materials having semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such metal oxides having semiconductor characteristics has been already known (for example, see Patent Documents 1 and 2).
In a semiconductor device like the thin film transistor, a breakdown (electrostatic breakdown) of the semiconductor device due to electrostatic discharge from the outside is a serious problem which causes reduction in reliability or productivity from the time of a manufacturing process and inspection of the semiconductor devices to the time of using them as products.
As electronic devices in which a thin film transistor is used, there are mobile devices such as a mobile phone or a notebook computer, and the like. For such a portable electronic device, power consumption which affects continuous operating time is a big problem. Also for a television set which is increasing in size, it is important to suppress the increase in power consumption associated with the increase in size.